Recently, an isolated-type AC-DC conversion power supply device (hereinafter, referred to as an isolated-type AC-DC converter) and an isolated-type DC-DC conversion power supply device (hereinafter, referred to as an isolated-type DC-DC converter) have been made smaller by increasing a switching frequency. However, when the device is made smaller, heat generation caused in accordance with loss of power conversion may more negatively affect the device. For that reason, it is desirable to increase power conversion efficiency, which is approximately 90% in the related art, to 95% or more. However, generally, if the switching frequency of a switching power supply increases, switching loss of the switching power supply increases, and thus, the heat generation increases.
To reduce the switching loss that leads to the heat generation, a technology of performing zero-volt switching (ZVS) according to a current resonance control method has been proposed. According to the current resonance control method, the switching frequency is modulated based on a feedback signal of an output power supply voltage, to maintain the output power supply voltage to be constant. Roughly speaking, a resonance state of a resonant circuit including a capacitor connected in series to leakage inductance on a primary side of an isolated-type transformer is controlled by modulating the switching frequency, and an output voltage on a secondary side of the isolated-type transformer is controlled by changing a voltage conversion coefficient. At this time, a time period (dead time) in which a high-side switching element and a low-side switching element are maintained to be turned off in common is provided, whereby a potential of a switching node is autonomously changed from a high potential to a low potential, or from a low potential to a high potential by action of a resonance current, in the dead time. After the autonomous transition of the potential is completed, the high-side or low-side switching element is turned on, and thus, the switching loss can be reduced and efficiency can be increased. This is called soft switching.
However, if timing to turn on the switching element is delayed, a diode connected in parallel with the switching element operates, and the switching loss occurs due to ON resistance thereof. In contrast, if timing when the switching element is turned on becomes fast, large primary noise may be generated by a hard switching operation, and simultaneously, a through current flows, so that large switching loss occurs. In the worst case, the switching element can be broken by heat which is generated due to the switching loss. That is, setting the dead time with an appropriate length is an important issue of an AC-DC converter and a DC-DC converter which employ the current resonance control method.
Meanwhile, time of the autonomous voltage transition of the switching node, which is performed by a soft switching operation, changes depending on a magnitude of the resonance current. For example, the resonance current changes according to feedback control of the output voltage, along with a change of a voltage of a primary power supply (supplying power supply) or a change of a load current of a secondary power supply (output power supply), and thus, the voltage transition time of the switching node changes every moment.